Now, a fair follow-up to this is that why don't we make text on the web into columns, so instead of big sidebars of white space you fit multiple columns of text on the screen? I'd say this is probably because the web uses scrolling instead of pagination. If there's more text than can fit on the page, it's faster and easier to just scroll down than it is to load a new page. But if you have a layout with columns, you'd have to scroll down to read, then scroll back to the top each time you finish a column, which is a hassle. So just having one long column requires the least user effort.
All said, of course some people might have different preferences. I'm sure some people find it easier looking from side to side with full screen text than scrolling down, or easier to click a next page button than scrolling down. But I think the trend for fixed width single columns on the web is one that's meeting many if not most peoples' preferences.
And by "wider" I mean from 1024x768 to 1080p, and by "bought" I mean "bought a decade ago".
There are definitely some people who just maximise everything out of habit, but I don't think there are enough to have created this trend of ultra-wide webpages.
Thankfully modern webpages being responsive at least somewhat fixes this issue, even if many sites still manage to screw this up.
What surprises me is how many users seem to keep their browsers maximized at all times, particularly on desktop-sized displays! I'm pretty sure I see people doing it more on Windows than on Mac for some reason, which makes me wonder if it's a UI design problem (of the OS, not websites).
As for maximising, Mac OS X never had a conventional one-click maximise button, the green button originally did something like resize the window to an optimal size (IIRC), and now it makes the app fullscreen on a separate "virtual desktop" of sorts. It might well be a habit from the times Windows didn't have Aero Snap.
Indeed, it's called "zoom". Fabulous when it works correctly, and one of my favorite features. You can still get at it with Option-Click (on the green), when the application supports it (not all do). Double-click on titlebar might also work, but I can't really check atm.
You can of course resize windows manually but it's tedious compared to a 2 button shortcut, especially having to hunt the 3 pixel wide resize button with wrist issues.
Remember when browsers supported user styles out of the box and put them in a somewhat discoverable location?
User-defined formatting hasn't worked since CSS was introduced.
Have you seriously tried yourself to read two identical blocks of text, one at full screen width (I tested 100% zoom Wikipedia on a 1440p, 27" screen at standard DPI) and one at a width of about 55 characters on the same screen? Have a go.
I find it easier to locate the next line after finishing one, which gives me a quicker reading speed.
Smaller widths make it easier for the "read the whole line in one glance" readers and is I think the main reason for this confusion.
Sometimes I like just glancing down a page and getting the gist of the screen. Other times I prefer to let my eyes do extra work and read denser text.